1. Field of the Invention
The present invention relates to a television tuner having a UHF tuner and a VHF tuner, and particularly to a television tuner which makes the selectivity characteristic of an input tuning circuit in the UHF tuner virtually uniform regardless of the channel from which signals are received.
2. Description of Related Art
FIG. 4 shows the configuration of a conventional television tuner which has a UHF tuner and a VHF tuner. As shown in the figure, UHF band television signals and VHF band television signals are inputted from an antenna (not shown) through an input terminal 31. The UHF tuner 40 and the VHF tuner 50 are connected to the input terminal 31 through a highpass filter 32.
The input tuning circuit 41 in the UHF tuner 40 has a varactor diode 41a and first and second inductance elements 41b and 41c which are serially interconnected and connected in parallel with the varactor diode 41a. The cathode of the varactor diode 41a is grounded through a d.c. cut capacitor 4d and one end of the first inductance element 4b is also grounded. The junction of the two inductance elements 4b and 4c is connected to the highpass filter 32 through a coupling capacitor 42.
The tuning frequency of the input tuning circuit 41 varies within a prescribed frequency range in the UHF band according to the tuning voltage Vt applied to the cathode of the varactor diode 4a. A UHF high frequency amplifier 43 is connected next to the input tuning circuit 41. Also, a mixer, etc., which is not shown in the figure, is also provided after the UHF high frequency amplifier 43.
On the other hand, the VHF tuner 50 also has an input tuning circuit, a VHF high frequency amplifier, a mixer or the like, though they are not shown in the FIG.
In this configuration, since the junction of the first inductance element 4b and second inductance element 4c of the input tuning circuit 41 is connected to the input terminal 31 through the coupling capacitor 42, as the impedance in the direction from this junction to the input terminal 31 is converted into a high impedance according to the ratio of the two inductances 4b and 4c and the impedance is connected in parallel with the input tuning circuit 41, and the tuning Q factor is thus determined.
In receiving UHF band television signals, the UHF tuner 40 is activated and the VHF tuner 50 is inactivated; the UHF band television signals selected by the input tuning circuit 41 are inputted to the UHF high frequency amplifier 43. The amplified UHF band television signals are converted into intermediate frequency signals by the mixer.
In receiving VHF band television signals, the VHF tuner 50 is activated and the UHF tuner 40 is inactivated; the VHF band television signals are selected and also converted into intermediate frequency signals by the VHF tuner 50.
It is well known that generally, the Q factor and bandwidth B of a tuning circuit that varies the tuning frequency by means of a varactor diode are expressed by equations Q=R/ωL and B=L×ω2/2πR, respectively. Therefore, even when the tuning impedance R (resistance) is constant, the bandwidth B broadens as the frequency increases. In addition, since the impedance of the coupling capacitor 42 in the above conventional tuner circuit configuration varies depending on the frequency, the tuning circuit's loss resistance R including the converted impedance is smaller as the tuning frequency is higher; as a consequence, variation in the bandwidth B is considerable and as shown in FIG. 5, the tuning characteristic curve which represents selectivity is steep (i.e. narrow bandwidth) and insertion loss S is large in UHF low band frequencies while the insertion loss is small and the tuning characteristic curve is gradual (broad bandwidth) in UHF high band frequencies.
Consequently, when low band UHF television signals are received, NF (noise FIG.) deteriorates; when high band UHF television signals are received, interference by television signals from a neighboring channel occurs.